PERISSODACTYLA 



181 



while the great group now classed under Equidae, comprising species from 

 the Lower Wasatch up to and including the living horses, is divisible into 

 four groups, or subfamilies, each containing several distinct phyla, which 

 stand in more or less direct ancestral relationship to those of the next 

 higher group, but between which there has not yet been found any species 

 which exactly bridge the gap between the subfamilies as now classified. 



The principal theories which may be said to be demonstrated in part, 

 but still lack convincing proof, are the polyphyletic origin of the order 

 and the use of dolicocephaly and brachycephaly for a standard of classi- 

 fication, as worked out by Osborn in the Titanotheres and Rhinoceroses. 

 The present proposed phylogeny of the rhinoceroses and horses, especially 

 as regards the derivation and true relationships of the subfamilies Hvra- 

 cotheriinae, Anchitheriinge, and Protohippinae, are problems in which 

 study and investigations should be continued. To this end further ex- 

 plorations should be made for more and better material, especially in the 

 Miocene and Pliocene formations, with a view to clearing up some of the 

 more important questions of classification and relationships of both the 

 horses and rhinoceroses of these periods. All the Eocene Perissodactyls 

 except the Titanotheres are in need of further revision and study, and 

 efforts should be made to secure additional material from the lower and 

 basal Eocene formations. This should eventually do much to clear up 

 the earlier ancestry of the great groups of the order, and give us a better 

 understanding of their true relationships one to the other. 



The importance of continued investigation in the Miocene, Pliocene, 

 and Pleistocene should perhaps be especially emphasized, for here, with 

 our modern methods of research, it now seems possible to trace at least 

 approximately the origin and development of the living species of horses 

 (including the asses and zebras) and rhinoceroses. 



CARNIVORA AND RODENTIA 

 BY W. D. MATTHEW 



CONTENTS p^gg 



Carnivora 182 



Material 182 



Results generally accepted 184 



Hypotheses under consideration 184 



Investigations desirable 185 



Rodentia 185 



Material 185 



Principal conclusions from the study of the material 186 



Most important investigations desirable in this line 186 



